1. Field of the Invention
This invention relates to a nitridation surface treatment that modifies the passive oxide layer on stainless steel alloys such that the interfacial contact electrical resistance is decreased and the corrosion resistance is improved. Stainless steel alloys processed according to the invention are useful in electrochemical energy conversion devices that require low interfacial contact electrical resistance and good corrosion resistance such as bipolar plates used in proton exchange membrane fuel cells.
2. Description of the Related Art
A major impediment to widespread commercialization of fuel cell technology is cost. One of the most expensive components in proton exchange membrane fuel cells (PEMFCs) are the bipolar plates, which serve to electrically connect the anode of one cell to the cathode of the next into a stack to achieve a useful voltage. They also separate and distribute reactant and product streams; to accomplish this, flow-field grooves are manufactured into the faces of the plates. Solid graphite is often used for bipolar plates, but is brittle and expensive to machine. Polymer/carbon fiber and carbon or graphite composite bipolar plates have shown promise; however, issues remain regarding their amenability to high-volume manufacturing techniques, performance, and the power densities achievable. U.S. Pat. No. 5,798,188 shows another type of bipolar plate in which a coating comprising metal, metal nitride or metal carbide is disposed on the surfaces of a polymer substrate.
Metallic alloys (e.g. stainless steels) would be ideal as bipolar plates because they are amenable to low-cost/high-volume manufacturing, offer high bulk thermal and electrical conductivities, and can be made in thin sheet or foil form (0.1-1 mm. thick) to achieve high power densities. However, the inadequate corrosion behavior of most metals in PEMFC environments has prevented their use. The key issues are passivation resulting in unacceptably high interfacial contact electrical resistance and dissolution of metallic ions, both of which can significantly degrade fuel cell performance.
There have been solutions proposed to the problem of unacceptably high interfacial contact electrical resistance in bipolar plates. For example, U.S. Pat. No. 6,379,476 describes a stainless steel product for producing a polymer electrode fuel cell in which discrete carbide/boride particles are added to the stainless steel such that the particles extend beyond the passive layer to lower contact resistance.
There has also recently been developed a nitridation treatment that forms a dense, continuous surface layer of CrN/Cr2N layers on Ni—Cr and related alloys. U.S. patent application Ser. No. 2003/0190515 discloses a corrosion resistant, electrically conductive component, such as a bipolar plate for a PEM fuel cell, that includes 20-55% chromium and balance base metal such as nickel, iron, or cobalt. The component has thereon a substantially external, continuous layer of chromium nitride. This continuous layer is typically on the order of 3-5 microns thick and behaves very well in PEMFC environments. These thermally grown Cr nitrides (CrN/Cr2N) on a Cr-bearing alloy, Ni-50Cr (wt %), show great promise in PEMFC bipolar plate environments. However, this alloy may be too expensive for certain PEMFC applications.
Thus, work has been undertaken to form similar nitride surface layers (as described in U.S. patent application Ser. No. 2003/0190515) on inexpensive, commercially available alloys. Similar thermal nitridation conditions have been applied to a relatively inexpensive, commercially available austenitic stainless steel sold under the designation 349. A discontinuous discrete mixture of CrN, Cr2N and (Cr, Fe)2N1−x (x=0 to 0.5) phase surface particles overlying an exposed austenite based matrix, rather than a continuous nitride surface layer, was formed. This resulted in unacceptably high corrosion rates under simulated PEMFC anodic and cathodic conditions. It is believed that many commercial stainless steels are degraded by the current nitridation conditions because they precipitate discrete internal Cr-nitride particles.
Therefore, there is still a need for metallic alloys, such as stainless steels, that can be used in PEMFC bipolar plates and that exhibit adequate corrosion resistance and acceptably low interfacial contact electrical resistance.